Hydrostatic displacement unit with reduced hysteresis

ABSTRACT

The invention relates to a displacement unit ( 1 ) of a hydraulic machine, for which the hysteresis is reduced. For this purpose, in the control spool ( 3 ) and/or at the control spool ( 3 ) a mass body ( 16 ) and a spring ( 17 ) are arranged, which are excited to resonance vibrations. The vibrations are self-excited and sustained by a partial flow rate of the hydraulic fluid which is modulated periodically. The high frequent vibrations are transmitted over the spring ( 17 ) onto the associated control spool ( 3 ), thereby reducing the friction and hence the hysteresis.

CROSS REFERENCE TO RELATED APPLICATION

Applicant hereby claims foreign priority benefits under U.S.C. §119 fromGerman Patent Application No. 102015218578.8 filed on Sep. 28, 2015, thecontent of which is incorporated by reference herein.

TECHNICAL FIELD

The invention relates to a hydrostatic displacement unit for thestepless adjustment of the displacement volume of a hydraulic machine.

BACKGROUND

Hydraulic machines with variable displacement volume or swallowingcapacity comprise a hydrostatic displacement unit which, for instance,sets the angle position of a swash plate or bent axis. This displacementunit comprises two essential elements for adjusting and controlling thedisplacement volume of the hydraulic machine. That is firstly thecontrol unit, which converts incoming mechanical, pneumatical,hydraulical or electrical control signals into adequate control volumeflow rates for the second essential element, the servo displacementunit, which engages with a displacement element of the hydraulicmachine. The control unit and the servo displacement unit are connectedto each other via fluid conducting control lines, which supply,respectively discharge the volume flow rates necessary for the servodisplacement unit. In order to set a certain displacement volume of thehydraulic machine against actions of internal spring forces and externaloperational forces, the control volume flow rates have to be suppliedunder adequate pressure. Such a displacement unit for hydraulic machinesis disclosed, for instance, in DE 10 2004 033 376 B3.

The control signals for the control unit are converted by actuators,preferably in axial force actions on the control spool. The signals canbe of various manners, for instance, mechanically,hydraulic-mechanically and as well as electrically. For the conversionof electric signals solenoids or switching magnets serves as actuators.Often the control unit is of a mechanical design with movable parts,which, for instance, are implemented as control valves comprising acontrol cylinder and a control spool designed to move longitudinally.The control spool usually is moved by actuators engaging with the samein axial direction. Naturally, friction is acting during the movement ofthese parts, which leads to a mechanical Hysteresis. Such a Hysteresisshows among others that equal control signals on the control unit causesdifferent control volume flow rates, respectively different controlpressures, depending on whether the control signal is set with anincreasing signal ramp or with a decreasing signal ramp. This is due tothe fact that the motion of the parts of the control unit show differentdirections according to the increasing or the decreasing control signalramp. Because of this, the friction forces act in different directionsand, mostly, also with different strengths.

In general, hysteresis are not wanted as these influence in a hardcontrollable manner the control volume flow rate to be adjusted by thecontrol unit according to given control signals, such that no uniquevalue can be associated to one single control signal for thedisplacement of a hydraulic machine, as it depends on the control rampwith which the control signal was set, respectively out of whichposition the control spool was displaced in the control cylinder.

For example, in control units with electric actuation of the actuator,the hysteresis of actual actuator force, respectively of the actuatorposition can be a counteracted in that the electrical control signal issuperimposed by an oscillation signal. This leads to a vibration of themovable parts of the actuator and therewith to a permanent high frequentreversal of the friction forces, which, for instance, are superimposedto the steady direction of forces resulting from the control signalramp. Herewith the influence of the static friction on the actuatoritself as well as on the actuated parts of the control unit,respectively on the control spool, is minimized, however, eventually,inaccuracies occur in the control of the control volume flow rates and,hence, of the displacement volume of the hydraulic machine if thecontrol spool is to be displaced with pulsing force. When applyingelectric control signals, dither or pulses width modulated (PWM) signalsare used, whose amplitude and frequency have to be adjusted to therequirements of a concrete design of the displacement unit. Pulse widthmodulated signals show the disadvantages vantages that their amplitudesdepend on the height of the electrical signals and, hence, are notoptimal in each and every control state. Dither signals are capable tohold the amplitudes in a broad but finally also limited band in aconstant and optimal manner independent from the height of theelectrical signal. However, not every amplitude, which, eventually, isoptimal for minimizing the Hysteresis, is also suitable for theelectronic control. Furthermore, it is difficult to apply aHysteresis-reducing oscillation signal, if the activation isnon-electric, respectively hydromechanic or pneumatic-mechanic.

In JPS62218676 (A) means for the reduction of hysteresis at a controlspool are described, with which pulsations in the hydraulic fluid areacting directly on a front face of a control spool via an amplifyingchamber arranged externally of a control unit and without interpositionof a further actuator, whose pressure serves as control signal for thecontrol unit. The pulsations or pressure fluctuations are created in theamplifying chamber by means of a mass oscillating on a spring. Thissystem is sophisticated as the amplifying chamber needs its own chargepump, and, furthermore, this results in increasing space requirementsfor the additional assembly groups.

SUMMARY

The invention is based on the object to provide a hydrostaticdisplacement unit initially mentioned, with which the mechanical causedhysteresis of the displacement unit, in particular of a control unitarranged in the displacement unit, can be reduced in a simple, howeverreliable and robust manner, without influencing therewith the height ofthe control signal in whatsoever manner. A further object of theinvention is to provide the possibility to retrofit displacement unitsof already existing hydraulic machines in a simple manner, withoutchanging the complete displacement unit in doing so.

The solution of this object is given such that the displacement unitcomprises a vibration unit, in particular an oscillation exciter, whichby means of excitation forces generates oscillations in the vibrationunit. Thereby the excitation forces are preferably independent from theactuator force, respectively the control signal. As the oscillationexciter is arranged directly at the control spool or at its mechanicalfeedback unit, these oscillations are transmittable as impulses to thecontrol spool. For this purpose the vibration unit/the oscillationexciter is attached directly to the control spool, for example, suchthat oscillations created in the vibration unit are transmittablemechanically, hydraulically, hydraulically-mechanically or as wellpneumatically to the control spool and/or to the actuator.

By means of the oscillations/impulses, in particular in longitudinaldirection of the excited control spool, the same receives a permanentlymotion reversal preferably, with small amplitude and, furtherpreferably, with high frequency. This permanent motion reversal issuperimposed to the relatively slow motion of the control spool due tothe control ramp. Under high frequency it is to be understood in thiscase that the oscillation motion of the control spool caused by thevibration unit runs quicker as the motion which is caused by theactuator forces acting on the control spool, and with which adisplacement of the hydraulic machine is obtained by means of conversionof the control signal, respectively the control signal pressure. Furtherpreferably the time constant (period) of the oscillation is shorter asthe displacement time of the control spool which is to be excited.Hence, to the uniform displacement of the control spool in the controlcylinder a kind of oscillation/vibration is superimposed. This vibrationleads to lower the static friction forces of the control spool by meansof the permanent motion reversal, whereby, at the same time, reducingthe Hysteresis effects.

A preferred embodiment of the inventive displacement unit can be consistin that the excitation forces are created by electrical, magnetical,electromagnetical, pneumatical or hydraulic forces. For instance, theexciting forces can be created by a mass body excitable to oscillation,which can be made fully or partially of magnetic material and, furtherexemplarily, is excited to longitudinal oscillations in that alternativecurrent is applied to inductive coils arranged thereon. Alternatively,such longitudinal oscillation of a mass body can be produced as well byelectro-friction or electro-mechanically in the manner of a (house door)bell.

A further preferred embodiment of the inventive displacement unitconsists in a hydraulic mechanic generation of the excitation forces.For this purpose the vibration unit comprises preferably a spring aswell as a mass body arranged in a cavity of the control spool andmovable in longitudinal direction of the spool. The mass body can beexcited to oscillations, for instance, by means of a hydraulic fluidflow acting on the same. The mass body transmits these oscillationsmechanically and/or hydraulically to the control spool, for instance, bymeans of the spring, which is force-locked to the respective controlspool. Alternatively, the exciting forces can be transmitted also byhydraulic fluid, which, for instance, acts on the front faces of theoscillating mass body. The hydraulic fluid is incompressible and hencesuitable for the transmission of forces, for example from a front faceof the mass body to an opposite cross wall in the cavity of the controlspool.

The frequency of the oscillation results, as commonly known, forinstance, from the mass of the mass body and the spring coefficient. Adamping of the oscillation due to friction forces, in particular due tothe viscosity of the hydraulic fluid covering the oscillating mass bodyleads to a shifting of the resonance frequency in direction to lowervalues and to a broadening of the resonance curve such that, inpractice, the frequency of the oscillation will be below the calculatedfrequency for the damping-free, idealistic case. Due to the damping ofthe oscillation a constant energy supply is necessary additionally, inorder to maintain the oscillation. Thereby, via the size of the partialflow, respectively the pressure in the hydraulic fluid, which issupplied to the vibration unit, the height of the oscillation frequencyand the amplitude, which is generated by the vibration unit can beinfluenced.

With the preferred integration of a vibration unit into the controlspool, a simple and effective possibility is provided to reduce theexisting hysteresis when adjusting the displacement volume of alreadyexisting hydraulic machines, wherein only the existing control spool hasto be interchanged with an inventive control spool.

A further simple and effective possibility to retrofit already existingdisplacement units to an inventive displacement unit, one can thinkabout to attach the vibration unit to a position feedback unit, forexample. Ideally, the oscillation exciter can transmit the oscillationto the element of the position feedback unit which engages mechanicallywith the control spool.

In general, one single vibration unit respectively oscillation exciterwhich is connected directly to the control spool is sufficient in orderto excite the control spool to oscillations and, hence, to lowereffectively the hysteresis with regard to the position in the controlcylinder. This is valid in particular with one part-control spools andalso if the vibration unit is supplied by charge pressure. Withoscillation exciters being excited by the control signal pressure, it isnecessary—in particular with two-side displaceable control spools—toprovide at each control spool side an oscillation exciter, as alwaysonly one side of the control spool is pressurized by control signalpressure and as only to one side of the control spool a control signalpressure is provided for pressurizing the servo displacement unit. If acontrol unit comprises more than one control spool, for each actuablecontrol spool a vibration unit is to be provided.

Further preferred, the vibration unit is designed such that theoscillations are self-excited. This means that except of applying acontrol signal or a connection to an energy source in form of a providedcharge pressure for the control unit, no further arrangements arenecessary to activate the oscillation, since this can be self-created.It shall be understood that energy losses of the oscillating mass bodyhave to be compensated, and which are given due to friction and by thedamping effect of the hydraulic fluid flushing around. As energy sourceserves, for example, a partial flow rate of the hydraulic fluid undercharge pressure branched-off of the hydraulic fluid channels for thecontrol signal or from the hydraulic fluid supply of the displacementunit, or, for example, from another pressure conducting line of thedisplacement unit or of the hydraulic machine, in general. For example,these fluid channel leads from an area under charge pressure to an areaunder low pressure. Hydraulic fluid under pressure serves for thecreation and sustainment of the excitation forces, wherein for thispurpose, for example, the oscillating mass body opens or closes fluidchannels arranged in the control spool. Analogously, the inventivevibration unit can be realized instead of hydraulically alsopneumatically.

The displacement unit according to the invention is provided for thereduction of friction and the hysteresis effects related therewith andcan be designed for an adjustment of the flow direction of the hydraulicworking fluid in the hydraulic machine in the two directions, wherebythe control unit can show only one or also two vibration units.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail in the following with the helpof embodiments which are depicted in the Figures. It is shown in:

FIG. 1 is a hydraulic machine with a displacement unit according to theinvention in a schematic view;

FIG. 2 is a schematic detail view of the vibration unit according toFIG. 1;

FIG. 3 is a cross-section of a hydrostatic displacement unit with avibration unit according to the invention;

FIG. 4 is a detail view of a cross-section of a further displacementunit according to the invention;

FIG. 5 is a cross-section of a control spool with a third embodiment ofa vibration unit according to the invention being in a first oscillationstate;

FIG. 6 is a cross-section of a control spool with a vibration unitaccording to FIG. 5 in a second oscillation state;

FIG. 7 is a cross-section of a control spool with a fourth embodiment ofthe vibration unit according to the invention in a first oscillationstate;

FIG. 8 is a cross-section of a control spool with a vibration unitaccording to FIG. 7 in a second oscillation state;

FIG. 9 is a cross-section of a control spool with a vibration unitaccording to FIG. 7 in a third oscillation state; and

FIG. 10 is a cross-section of a control spool with a vibration unitaccording to FIG. 7 in a fourth oscillation state.

DETAILED DESCRIPTION

FIG. 1 shows a hydraulic machine 27 adjustable in two conveyingdirections, with a displacement unit 1 according to the invention in aschematic view. The hydraulic machine, which can be a hydraulic motor ora pump, comprises a driving or driven shaft 28 which, for instance, fora pump is driven by a not shown combustion engine. In this case, thepump conveys via lines 29 a fluid to and from a consumer. Thereby, aservo displacement unit 30 serves for the adjustment of the displacementvolume and the conveying direction of the hydraulic fluid, which, forinstance, can be done by varying the displacement angle of a swash plateor a bent axis of the pump. The control of the servo displacement unit30 is effected over a control unit 2 with two control cylinders 4, ineach of which a control spool 3 is mounted movable longitudinally.Thereby, both control spools 3 are connected rigid to each other suchthat a displacement of one control spool 3 causes the displacement ofthe other control spool 3 as well. The control spools 3 are displaced inthis embodiment by two proportial magnets used as actuators according tocontrol commands of a not shown control electronic of the hydraulicmachine 27, whereby via lines 32, 33 fluid flows provided by a chargepump 31 are guided to and from the servo displacement unit 30 or to atank 50 of the hydraulic machine 27. These details of the design and theoperation of a hydraulic machine 27 are known to a person skilled in theart such that further details can be omitted. For this purpose it shouldbe indicated merely that the actuators 5 for the displacement of bothcontrol spools 3 can be mechanic, pneumatic, electric or hydraulicmeans.

According to the invention the control spools 3 are operativelyconnected mechanically with an oscillation exciter as vibration unit 8.This vibration unit 8 is provided for bringing the control spools 3 inlongitudinal oscillations, i.e. in oscillations parallel to itsdirection of movement within the control cylinders 4. Hereby, thehysteresis in the responding behavior of the control unit 2 iseliminated nearly completely, at least reduced significantly. Thevibration unit 8 is configured exemplarily as a resonance oscillatorhaving a mass body 16 and a spring 17, which are arranged in a cavity15, and which are mounted movable in the longitudinal direction of thecontrol spool 3 (see FIG. 2). The cavity 15 is connected via a line 34to charge pump 31 and via a line 35 with tank 50. For further details ofthe operation of such a vibration unit 8, it is referred to thedescription of FIGS. 2 and 4.

It shall be understood that instead of the vibration unit 8 shown inFIGS. 1 and 2, another kind of oscillation exciter 8 can be attached tothe control spool as well. Such a control spool must not necessarily bein direct mechanic contact with a control spool 3, for example, can becoupled to the associated control spool 3 via a stationary or variablemagnet field. Likewise, also piezoelectric or magnetostricitveoscillation exciters can be considered to be integrated in thecorresponding control spool 3, for example.

FIG. 1 shows exemplarily a hydraulic machine 1 adjustable in bothdirections, in which the displacement unit 1 with the control unit 2 andthe servo displacement 30 is designed, in general symmetrically. Itshall be understood, however, that the inventive displacement unit canbe applied also to a hydraulic machine 27 which can be adjusted only inone conveying direction. Here, the vibration unit 8 is only effective atone single control spool 3.

With the description of the following Figures all reference numerals forindicating the same constructive features are remained. Here, it is tobe annotated for ensuring clarity single parts or elements are indicatedwith only one reference numeral even though if they are shown severaltimes. In FIG. 2 a schematic detail view of an inventive vibration unit8 according to FIG. 1 is shown. Here, a resonance oscillator is shownhaving a mass body 16 and a spring 17, which are arranged in a cavity15, and which are mounted movable in longitudinal direction of cavity15. The cavity 15 is connected via a line 34 with the charge pump 31 andvia a line 35 with tank 50. In the mass body 16 a longitudinal channel18 is formed, which ends in direction of a cross wall 22 of cavity 15. Across wall 22 defines together with the opposite arranged front face 26of mass body 16 a chamber 36, whose volume is variable according to theposition of mass body 16.

From the longitudinal channel 18 two cross bores 19 and 19 a arebranched off, which leads to an outer side of mass body 16. The chamber36 can be connected via the cross bores 19 and 19 a as well as vialongitudinal bore 18 with lines 34 and 35 hydraulically, wherein theline 34 comes from the charge pump 31 and the line 35 leads to tank 50.The lines 34 and 35 are arranged spaced from each other in the chambersuch that by the displacement of the mass body 16 in cavity 15 alwaysonly one of both lines 34, 35 overlaps with one of the cross bores 19,19 a.

The way of function of vibration unit 8 is as follows: In the actualstate shown in FIG. 2 of the vibration unit 8 the cross bore 19 overlapswith the opening of line 34, which is forced with hydraulic fluid undercharge pressure by charge pump 31. The pressure fluid enters intochamber 36 over the longitudinal channel 18 and acts on the front face26 of mass body 16. Hereby, the mass body 16 is displaced against theforce of spring 17 in direction to the spring. With sufficientdisplacement of the mass body 16, the overlap with cross bore 19 andline 34 ends. Instead of this, the second cross bore 19 a reachesoverlap with the opening of line 35, which leads to tank 50 under lowpressure. The pressure in chamber 36 is relieved, whereupon the spring17 moves the mass body 16 in direction to cross wall 22. With the newlyoverlap of cross bore 19 with line 34, the pressure in chamber 36 risesagain, whereupon the described procedure repeats. Hence, a oscillationof the mass body 16 in cavity 15 occurs, whose frequency is determinedin known manner by the mass of the mass body 16, the pressure of thehydraulic fluid, which flows over line 34 into chamber 36, and thespring coefficient of spring 17. This frequency can be reduced inpractice by the viscosity of the pressure fluid and further frictioneffects. The oscillation is transmitted, for example, via spring 17 tothe wall of cavity 15, on which the same is supported, and hence, can beused for the generation of oscillation of the control spool 3 coupledwith the same. One can imagine, that front face 26 of mass body 16driven by the spring 17 abuts at cross wall 22 in the same manner, asthe opposite front face of front face 26 can abut at a bottom surface ofcavity 15 if the pressure guided into the chamber 36 via lines 34 movesthe mass body 16 from the position shown in FIG. 2 towards the right.Naturally, an alternating abutment of the mass body 16 on cross wall 22and on bottom surface 37 are covered by the inventive idea as well.

The kind of vibration unit 8 described above is self-excited as thepressure fluid supplied via line 34 leads to the motion of mass body 16to the right. Thereby its motions are powered until a stationary stateis reached, which is sustained by the interplay of supply and dischargeof pressure fluid to and from chamber 36.

In FIG. 3 a hydrostatic displacement unit 1 with a vibration unit 8according to the invention is shown partially in cross section. Shown isonly the control unit, which provides the servo displacement unit 30(not shown) with hydraulic fluid under control pressure. The controlunit 2 shows a longitudinal bore forming the control cylinder 4. In thecontrol cylinder 4 a symmetrically formed two-sided control spool 3 isarranged moveable longitudinally. The lever system of a positionfeedback unit 40 engages in a central portion of the control spool 3,which causes a neutral position of control spool 3. The way of operationof such a position feedback unit 40 in a displacement unit 1 isdescribed in DE 10 2004 033 376 B3, for example, and is known to aperson skilled in the art. For example, on pointer 42, which engageswith control spool 3, a vibration unit 8 is arranged such that thedirection of amplitude of the oscillation exciter is generallyperpendicular to the pointer longitudinal direction. When retrofittingan existing displacement unit according to FIG. 2 a vibration unit 8 ofthe kind of a door bell would be preferred, however, all other kinds ofoscillation-excitation are possible as well, and hence, covered by theinventive idea.

At the front faces of the control spool 3, which is shown in FIG. 3,actuators 5 engages exemplarily in form of proportional solenoids 5 viaplungers 6, which according to the present control signal cause adisplacement of the control spool 3 in control cylinder 4. Thereby theoverlap of the control edges 9 which are formed in the control cylinder4 and by ring grooves 10 and ribs 11 formed in control spool 3 ischanged, which leads to an adjustment of the control pressure ascommonly known, and which is guided to the servo displacement unit 30.The supply of the control unit 2 with hydraulic fluid and its dischargeto servo displacement unit 30 or to a tank 50 of the hydraulic machine27 are effected over fluid channels 7, 24, 25 which are shown in FIG. 3only exemplarily.

In another preferred embodiment of the invention the vibration unit 8 isarranged in control spool 3. The vibration unit 8 comprises a spring 17and a mass body 16 arranged in a cavity 15 of control spool 3. The massbody 16, the spring 17 and the control spool 3 are force-lockedconnected to each other and, hence, form a construction which is capableto oscillate. The mass body 16 is guided slidably in cavity 15 such thatit can oscillate freely apart from the damping caused by the hydraulicfluid surrounding it. Further details of the exemplarily describedcontrol spool 3 with integrated vibration unit 8 are shown in FIG. 4.

FIG. 4 shows a detailed view of the displacement unit 1 according toFIG. 3 in cross-section. Shown is one end region of the control cylinder4 with an end region of the control spool 3 guided therein slidably. Inthe inner wall of control cylinder 4 ring grooves 10 are formed whichare separated by ribs 11. Together with the circumferential ring grooves23 on the control spool 3 control edges 9 are formed hereby, which, ascommonly known, determine the height of the control pressure reaching atthe servo displacement unit 30. The hydraulic fluid under pressure issupplied for this purpose via a supply fluid channel 7 (see FIG. 3) andis discharged via the low pressure channel 24 to a tank 50, for example.

In a cavity 15 of the control spool 3 an inventive vibration unit 8 isarranged which, for instance, consists of a mass body 16 and a spring17. A discharge bore 14 in the control spool 3 leads out of cavity 15 toa discharge outlet 24 under tank pressure, for instance. A cavity 15 isclosed on the opposite side with a cross wall 22. The mass body 16comprises a longitudinal channel 18, which crosses the same in directionof the longitudinal axis 13 of control spool 3. From longitudinalchannel 18 a continuing cross bore 19 branches off which enters in asupply bore 21 in control spool 3. This supply bore 21 formed in thewall of cavity 15 of control spool 3 leads to the area of the fluidchannel 7 respectively to a ring groove 10 communicating with the same.Thereby, the supply bore 21 is arranged such that it can be aligned atleast partially or time partially with ring channel 23 and cross bore 19in the mass body 16, this is determined in each case by the actualposition of the mass body 16 in the cavity 15. The discharge bore 14,the longitudinal channel 18 with cross bore 19 and the supply bore 21form altogether a fluid channel, which leads from the supply fluidchannel 7 via the ring groove 10 to the low pressure channel 24.

The way of operation of the integrated vibration unit 8 shownexemplarily, is as follows: The hydraulic fluid under charge pressurecoming from the supply channel 7 acts via the supply bore 21 in controlspool 3 and via the cross bore 19 in mass body 16 onto the front face 26of mass body 16 in cavity 15, and causes a displacement of mass body 16against the force of spring 17 such that the overlap between the crossbore 19 and the supply bore 21 diminishes. Via the longitudinal channel18 in mass body 16 and the discharge bore 14 in control spool 3, thepressure in chamber 36 can be relieved, whereby the hydraulic force onthe mass body 16 decreases. If the hydraulic force on mass body 16 havebeen lowered to a value lower as the spring force of spring 17, thespring 17 moves the mass body 16 again in direction to the distal end ofcontrol spool 3. Hereby the overlap of cross bore 19 with supply bore 21increases until the mass body 16 abuts at the cross wall 22, forexample. Subsequently, the pressure in chamber 36 increases again andthe mass body 16 is displaced again in direction to spring 17 if thehydraulic force on its front face 26 is high enough. This again causesthe closure of the passage from supply bore 21 to cross bore 19whereupon the pressure in cavity 15 decreases and the spring 17 movesthe mass body 16 again towards the cross wall 22. This procedure isrepeated periodically, which leads to the sustainment of the generatedoscillation. Hereby, losses due to friction and damping due to theviscosity of the hydraulic fluid as well as due to the forces acting onthe control spool 3 are compensated such that the oscillations arerunning in general with constant amplitude, once they have started. Thisprocedure shows as well that the oscillation of the mass body 16 isself-excited.

The oscillating mass body 16 is connected via the spring 17 with thecontrol spool 3 in a force-locked manner. The oscillation forces of themass body 16 are transmitted via the cross wall 22 or the bottom surface37 of cavity 15 onto the control spool 3 such that the same oscillatesalso in the tact of the high frequent oscillation of mass body 16. Thisoscillation superimposes the slower motion of the control spool 3, whichacts under the influence of the control forces effected by the actuators5. These oscillations of the control spool leads to a reduction of thefiction forces, for instance, with the control cylinder wall, as herebyat least the initial friction is eliminated, and hence, the soughtreduction of the hysteresis is achieved. For the person with skills inthe relevant art it can be seen that hydraulic forces which cause in theabove given embodiment an oscillation of the mass body can be,correspondingly, in an analogous way also electric, mechanic, pneumaticor magnetical forces. Here, the working principal of a house door belldriven by means of a relay serves as a figurative example.

FIG. 5 shows a cross-section through a control spool 3 having a thirdembodiment of a vibration unit 8 according to the invention and being ina first oscillation state. The control spool 3—as before—is guided inthe control cylinder 4 movable longitudinally. The control spool 3 isactuated by a not shown actuator, which engages at the front face 12 ofcontrol spool 3 or, alternatively, at a cap 60. For reasons of clarity,the control spool 3 is shown in this and all further Figures in asimplified manner. The ring channels, the passages and control edgesusually formed within the same are not shown, however, are supposed tobe existent, as they are common in the art.

The vibration unit 8 is arranged in a longitudinal bore 51 of controlspool 3. The vibration unit 8 comprises a plunger 52 on which a bushing53 is guided movable longitudinally. The bushing 53, however slidable,abuts sealed with its end regions 67 at the inner wall of longitudinalbore 51. The displacement range of bushing 53 is limited with regard toplunger 52 by stoppers, for example, in form of wire rings 54 which arearranged in the end regions of plunger 52.

From bottom 52 of longitudinal bore 51 a channel 56 leads via a dynamicpressure orifice 57 to the discharge outlet 58, which conducts hydraulicfluid to the not shown tank 50 of the hydraulic machine 27. At theopening of the channel 56 in bottom 55 of the longitudinal bore 51, aseat 59 is formed which, in interaction with the plunger 52, closes thechannel 56; this is shown in FIG. 5. The other end of longitudinal bore51 is closed with a cap 60 in which a channel 61 with a dynamic pressureorifice 62 is formed. This channel 61 also leads to tank 50, which isnot shown here. In cap 60 there is also a seat 63 for the plunger 51such that the channel 61 is closable liquid-tight by the plunger 52.However, channel 61 in the oscillation phase shown in FIG. 5 is openedfor the passage of fluid, whereas the channel 56 is closed.

The outer walls of bushing 53 comprise a region 68 in the sectionbetween the two end regions 67 having a smaller diameter. In theproximity of the end regions 67, cross bores 69 are formed in bushing53, from which oil supply orifices 70 lead to cavities 71 which areformed on both sides of bushing 53 in the longitudinal bore 51. A crossbore 72 in control spool 3 connects with region 68 with ring groove 10arranged in the control cylinder 4, for feeding hydraulic fluid undercharge pressure such that pressure fluid supplied via the fluid channel7 can reach the cavity 71 via the oil supply orifice 70.

In FIG. 6 a cross section through a control spool 3 having a vibrationunit 8 according to FIG. 5 is shown in a second oscillation state. Inthis phase of the oscillation of the vibration unit 8, the plunger 52and the bushing 53 are shown in a second end position. The plunger 52 isdisplaced to the right and abuts with its end region on the seat 63 ofcap 60. Hence, the channel 51 leading to tank 50 is closed, whereaschannel 56, also leading to tank 50, is open.

The working principle of the vibration unit 8 according to thisembodiment is as follows: In the state shown in FIG. 5, pressure fluidunder charge pressure flows from the ring groove 10 via the cross bore72, via the region with lower diameter 68 and via the cross bores 69 aswell over the oil supply orifices 70 arranged on both sides of thebushing, to the cavities 71 of longitudinal bore 51 in control spool 3.Thereby, in the left-side cavity 71, a higher pressure is built up, asthe channel 56 is closed by the plunger 52. The higher pressure on theleft-side front face of bushing 53 moves the same to the right. Theplunger 52 receives left-side lifting forces and on the right-sidepressing forces by the pressure generated by the dynamic pressureorifice 62, which exceeds the lifting forces and, hence which press theplunger 52 onto the left-side seat 59. The bushing 53, which abutedbefore on left-side wire ring 54, now contacts the right-side wire ring54. By this continuing pressurizing of the left-side cavity 71 and thekinetical energy, bushing 53 takes along plunger 52 during its motion tothe right. This causes that plunger 52 lifts from the left-side seat 59and, hence, opens the channel 56 to tank 50. Shortly afterwards theplunger 52 abuts on the right-side seat 63 and, closes therewith channel61 to tank 50. This is the state shown in FIG. 6. Now, the procedurerepeats, however in the opposite direction, as the pressure building upin the right-side cavity 71 is now higher than the pressure of theformer filled left-side cavity 71, which is released now via channel 56.Hereby, the bushing slides in the longitudinal bore 51 of control spool3 to the left. When bushing 53 abuts on the left wire ring 54 it takesalong plunger 52 until this closes again channel 56.

This alternating opening and closing of the channels 56 and 61 leadingto tank 50 caused by the motion of bushing 53 and taking along plunger52 results in a periodical inversion of the direction of the motion.Hereby, the abutment of the plunger on the respective seat 59 or 36exerts an impulse on the control spool 3, which, hence, is excited to aforced vibration. The oscillation is self-excited as the displacement ofthe plunger 52 and the bushing 53 can be excited by feeding pressurefluid to supply channel 7. The frequency of the generated oscillationcan be set by the dimensioning of the single components of the vibrationunit 8. Hereby, the height of the charge pressure, the mass of theplunger and the bushing, and its dimensioning as well as their crosssections and lengths of the participating channels and orifices as wellas the viscosity of the hydraulic fluid are influencing variables.

FIG. 7 shows a cross section through a control spool 3 comprising afourth embodiment of a vibration unit 8 according to the invention in afirst oscillation state. The FIGS. 8 to 10 show the same embodiment infurther phase of the oscillation. This also preferred embodiment showsin general a similar construction of the vibration unit 8 as the oneshown in the FIGS. 5 and 6. Therefore, the same elements are indicatedwith the same reference numerals. In the longitudinal bore 51 of thecontrol spool 3, again a bushing 53 is guided movable longitudinally. Itcomprises, as before, to end regions 67, between which a region 68 oflower diameter is present. In the center of this region 68 radialpassages 75 oriented towards the inner of bushing 53 are formed. Theplunger 52 is arranged in a passing-through longitudinal bore of thebushing 53 movable longitudinally. The plunger 52 is formedsymmetrically and shows right and left of its center a ring groove 81 aswell as radial cross bores 76 crossing the plunger. From these crossbores 76 longitudinal channels 77 starts at the respective front faces78 which discharge at the respective front faces 78 of plunger 52. Thebottom 55 of the longitudinal bore 51 in control spool 3 is designed inits diameter such that a region with lower diameter forms a stop 79 forthe left-side medial front face 80 of the bushing 53. On the oppositeside of longitudinal bore 51 a correspondent stop 82 is formed on cap60. The bushing 53 is shorter than the plunger 52 whose movement rangeis limited by the bottom of the longitudinal bore 51, respectively bythe cap 60. Hence, the runnable displacement path of bushing 53 islonger than the one of plunger 52. In each of the bottom of thelongitudinal bore 51 and the control spool 3 as well as in the cap 60 adynamic pressure orifice 57, respectively 62, is formed which leads tothe not shown tank 50.

In the state of the vibration unit 8 shown in FIG. 7, which can be seenas initial state of the oscillation cycle, the plunger 52 and thebushing 53 are situated at its left-side end position of its movement.The front face 78 of plunger 52 abuts at the bottom 55 of thelongitudinal bore 51, front face 80 of bushing 53 on stopper 79.Pressure fluid under charge pressure flows from the charge pressure port7 via the ring groove 10 and the cross bore 69 in control spool 3 intothe region 68 of the bushing 53 with lower diameter. From there, thepressure fluid reaches via passages 75 the left ring groove 81 ofplunger 52 and, further, via the cross bore 76 and longitudinal channel77—stepped in its diameter in the sense of an orifice—to front face 78of plunger 52. This front face 78 comprises a cross channel 83 via whichthe inward flowing pressure fluid can enter cavity 84 present in thebottom 55 of the longitudinal bore 51. Thereby, a pressure is built upin cavity 84, which acts on the respective front faces 70, 80 of plunger52 and bushing 53. Both elements move under the effect of the pressuretowards the right.

This motion state is shown in FIG. 8. The plunger 52 already has beenlifted from the bottom of the longitudinal bore 51 as well as bushing 53left the stopper 79. The ring groove 81 in plunger 52 still overlapswith the passages 75 of bushing 53 such that pressure fluid can stillflow to the left side, wherein the dynamic pressure orifice 57 takescare of the generation and sustainment of sufficient pressure in theleft-side cavity 84 of longitudinal bore 51. This pressure moves theplunger 52 and the bushing 53 further towards the right, wherein theplunger 52 runs ahead of plunger 53, as it is shown in FIG. 9, as thesurface area of its front face 78 is bigger than the one of front face80 of bushing 53. Preferably, its mass is lower too than the one ofbushing 53.

In the state of oscillation shown in FIG. 9, the right-side front face78 of the plunger 52 has reached already stop 82 at cap 60 and,therewith, is at the end of its displacement path. The bushing 53 indeedmoved also further to the right, however, an overlap of the passages 75with the left side ring groove 81 of the plunger 52 still exists. Hence,pressure fluid still flows into the left-side cavity 84 of thelongitudinal bore 51. The pressure in cavity 84 moves the bushingfurther towards the right until the same abuts with its right front face80 on the right stop 82. This right-side end state is shown in FIG. 10.

The state shown in FIG. 10 corresponds in general to the left-sideinitial state according to FIG. 7, however, with the difference that nowthe passages 75 of the busing 53 overlaps with the right-side ringgroove 81 of the plunger 52. Thereby, pressure fluid flows intoright-side cavity 85 of longitudinal bore 51 such that an inversion ofthe direction of movement of the plunger 52 and the bushing 53 occurs.Hereby, both elements run through the same states as it is shown inFIGS. 7 to 9, however, in inverted direction, whereupon an inversion ofdirection newly takes place. Hence, a periodical alternate motion ofplunger 52 and bushing 53 is created; with other words, an oscillation.As both elements transmit an impulse onto the control spool 3 whenimpacting the bottom 55 of the longitudinal bore 51, respectively on thecap 60, the oscillation is transmitted on the control spool 3, whichleads to the intended reduction of hysteresis. Even if both elements,plunger 52 and bushing 53, do not impact on bottom 55, the forcedinversion of direction causes already vibration-stimulating impulseswhich can be transmitted by the hydraulic fluid onto the control spool3.

The self-excitation of the oscillation and the determination of itsfrequency is done in the same way as with the embodiments describedbefore.

While the present disclosure has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisdisclosure may be made without departing from the spirit and scope ofthe present disclosure.

What is claimed is:
 1. A displacement unit of a hydraulic machine forthe stepless adjustment of the displacement volume, having a controlvalve by means of which hydraulic fluid under control pressure can beguided to a hydraulic servo displacement unit, wherein, caused by anactuator or by a direct set pressure signal, a displacement of a controlspool arranged slidable in the control valve sets the height of thecontrol pressure, which can be guided to the servo displacement unit fordisplacing a displacement element, wherein, the displacement unitcomprises an oscillation exciter which can be set into vibrations bymeans of excitation forces which are independent from the force of theactuator, wherein the oscillation exciter is arranged directly at thecontrol spool or at a position feedback unit which is mechanicallycoupled to the control spool such that the vibrations are transmittableto the control spool.
 2. The displacement unit according to claim 1,wherein, the excitation forces are generatable by hydraulic, pneumatic,mechanic, electric or magnetic means.
 3. The displacement unit accordingto claim 1, wherein the oscillation exciter is integrated into thecontrol spool.
 4. The displacement unit according to claim 1, whereinthe oscillation exciter is coupled with a movable element of theposition feedback unit.
 5. The displacement unit according to claim 1,wherein the oscillation exciter comprises a mass body movable in acavity, and a spring, wherein the mass body is excitable mechanically,electrically, hydraulically, magnetically or pneumatically tovibrations, which are transmittable to the control spool mechanically orhydraulically.
 6. The displacement unit according to claim 1, wherein ahydraulic fluid from a pressure supply of the displacement unit servesfor the creation and sustainment of the excitation forces.
 7. Thedisplacement unit according to claim 1, wherein hydraulic fluid from thecontrol pressure supply of the servo displacement unit under controlpressure generated by the displacement unit serves for the creation andsustainment of the excitation forces.
 8. The displacement unit accordingto claim 1, wherein hydraulic fluid under pressure from an externalhydraulic control signal generator serves for the creation andsustainment of the excitation forces.
 9. The displacement unit accordingto claim 1, wherein the oscillation exciter is self-excited.
 10. Thedisplacement unit according to claim 5, wherein the mass body isforce-locked connected via the spring to the control spool and whereinduring operation the mass body opens and closes fluid channels arrangedwithin the walls of the cavity in an oscillating manner.
 11. Thedisplacement unit according to claim 10, wherein the fluid channels areleading from a region under control pressure or servo pressure to aregion of the displacement unit under a low pressure.
 12. Thedisplacement unit according to claim 1, wherein the displacement unit isdesigned for the adjustment of the conveying direction of the hydraulicmachine in two directions, and wherein for each conveying direction anoscillation exciter is associated, and wherein just one of the twooscillations exciters can be activated.
 13. The displacement unitaccording to claim 1, wherein the valve oscillations created by theoscillation exciter act in axial direction onto the control spool. 14.The displacement unit according to claim 1, wherein the oscillationexciter comprises a plunger and a bushing arranged on the plunger, whichare arranged longitudinally movable relative to each other and relativeto the control spool in the longitudinal bore.
 15. A control spool for adisplacement unit of a hydraulic machine with an oscillation exciterwhich can be set into oscillations by excitation forces and which isarranged directly at the control spool such that the oscillations aretransmittable to the control spool.
 16. The control spool according toclaim 15, wherein the oscillation exciter is integrated in a cavity ofthe control spool.
 17. The control spool according to claim 1, whereinthe oscillation exciter comprises a mass body arranged movable in acavity of the control spool, and a spring 1, wherein the mass body isexcitable mechanically, electrically, hydraulically, magnetically orpneumatically to oscillations, which are transmittable to the controlspool mechanically or hydraulically.
 18. The control spool according toclaim 17, wherein the mass body is force-locked connected via the springto the control spool, and wherein by means of the movements of the massbody fluid channels arranged in the walls of the cavity can be opened orclosed periodically.
 19. The control spool according to claim 15,wherein the oscillation exciter is self-excited.
 20. The control spoolaccording to claim 19, wherein the oscillations created by theoscillation exciter act in axial direction onto the control spool. 21.The control spool according to claim 15, wherein the oscillation excitercomprises a plunger and a bushing arranged on the plunger, which arearranged in the control spool longitudinally movable relative to eachother and to the control spool arranged in a longitudinal bore.